1. Field of the Invention
The present invention is a technology that relates to an exposure apparatus for exposing a substrate via a projection optical system and a liquid, and a device fabrication method using such an exposure apparatus.
2. Description of the Related Art
A conventional lithography process for fabricating a semiconductor device, a liquid crystal display device, or the like, uses an exposure apparatus for transferring a pattern (which may be called a “reticle pattern”, hereinbelow) formed on a mask or a reticle (generally called a “mask”, hereinbelow) via a projection optical system onto a substrate such as a wafer or a glass plate which is coated with a resist or the like. Generally, a still exposure type exposure apparatus (i.e., a stepper) or a scanning type exposure apparatus (a so-called scanning stepper) is generally used.
In such an exposure apparatus, it is necessary to reliably project a pattern formed on the reticle to the substrate with high resolution. Therefore, a method for adjusting the projection optical system has been proposed, in which aberrations remaining in the projection optical system are measured by using, for example, a wavefront aberration measurement technique disclosed in Reference Document 1 (Japanese Published Unexamined Patent Application No. 2002-202221), so as to obtain desired imaging performance and optical characteristics.
There has been a demand in recent years for projection optical systems having higher resolution in order to handle the much higher levels of integration of device patterns. The shorter the exposure wavelength used and the larger the numerical aperture of the projection optical system, the higher the resolution of the projection optical system. Consequently, the exposure wavelength used in exposure apparatuses has shortened year by year, and the numerical aperture of projection optical systems has increased. Furthermore, the mainstream exposure wavelength is currently the 248 nm assigned to KrF excimer laser, but an even shorter wavelength of 193 nm assigned to ArF excimer laser is also being commercialized. In addition, as with resolution, the depth of focus (DOF) is important when performing exposure. The following equations express the resolution R and the depth of focus δ, respectively.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
Therein, λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. Equations (1) and (2) teach that if the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ decreases.
If the depth of focus δ becomes excessively narrow, then it will become difficult to make the relevant surface of the substrate coincide with the image plane of the projection optical system, and there will be a risk of insufficient margin during the exposure operation. Accordingly, a liquid immersion method has been proposed, as disclosed, for example, in Reference Document 2 (the booklet of PCT International Publication No. WO99/49504), as a method to substantially shorten the exposure wavelength and increase the depth of focus. In this liquid immersion method, a gap between the lower surface of the projection optical system and the surface of the substrate is filled with a liquid, such as water or an organic solvent, thus forming a liquid immersion area and taking advantage of the fact that the wavelength of the exposure light in a liquid is 1/n that of in air (where n is the refractive index of the liquid, normally about 1.2 to 1.6), thereby improving the resolution as well as increasing the depth of focus by approximately n times.
As far as is permitted by the relevant national law or regulations, the disclosures of the Reference Documents are each hereby incorporated by reference.
A wavefront aberration measurement method applied to a projection optical system of a liquid immersion exposure apparatus should be performed via a liquid provided between the projection optical system and a wavefront aberration measurement device. If the liquid for exposure leaks and enters inside the wavefront aberration measurement device, the liquid may cause problems such as a failure of the measurement device or leakage of electricity. In such a case, aberrations may not be correctly measured, and the projection optical system may not be controlled, so that exposure operation cannot be preferably performed.